Removal of water is a key issue to be addressed in synthesis gas conversion reactions. For instance, water is a primary by-product in a Fischer-Tropsch (FT) reaction and its presence is generally detrimental to the overall efficiency of the FT reaction. In an FT reaction, a synthesis gas mixture of carbon monoxide (CO) and hydrogen gas (H2), referred to hereinafter as “syngas,” is converted in the presence of an FT catalyst (most commonly iron- or cobalt-based) into hydrocarbon products, water and other byproducts. The syngas may be generated from a number of carbon containing sources such as natural gas, coal or bio-mass. It is often desirable to convert these carbon sources into a liquid hydrocarbon mixture from their original gas or solid states.
As the FT reaction occurs at relatively high temperature, the water produced is generally in the form of water vapor. Produced water vapor reduces the partial pressures of FT reactants, thus affecting reaction kinetics and reducing reaction rates. Water vapor is also detrimental to the life of FT catalysts, and especially in high partial pressures, leads to the oxidation of the catalyst and the sintering of the catalyst support, resulting in a reduction in the catalyst activity. Due to these adverse effects of water on the FT reaction, conventional FT fixed bed reactors have a relatively low rate of per pass CO conversion to limit high water partial pressures in the reactor. Conventional FT fixed bed reactors separate water from other reaction products and unreacted CO and H2 gas after they exit the reactor's outlet. The unreacted CO is often recycled back to an FT reactor inlet so that it may again potentially be converted into a hydrocarbon, at the cost of increased throughputs, resulting in larger reactors.
Efforts with respect to in situ dehydration in conversion of syngas to hydrocarbon products and water have been described. U.S. patent application Ser. No. 12/342,799 (Fayyaz-Najafi et al.), assigned to Chevron U.S.A. Inc., hereby incorporated by reference in its entirety, describes improved designs for FT reactors, in which water is removed in situ using a membrane and wherein heat management issues are also addressed.
Another issue to be addressed in synthesis gas conversion reactions is control of the ratio of hydrogen to carbon monoxide (H2/CO) in the syngas, as this affects the product distribution. When this ratio is too high, reaction products include undesirably high levels of methane and light gas. When this ratio is too low, reaction products include undesirably high levels of olefin and oxygenates. Additionally, consumption of hydrogen in the FT reactor occurs rapidly in the initial or upstream section of the reactor thereby lowering the partial pressure of hydrogen and thus the reaction rate and the H2/CO ratio in the downstream section of the reactor. Although the downstream end of the reactor has available heat removal capacity, this capacity remains unused when this section of the reactor is hydrogen starved.
It would be desirable to provide an improved process for the in situ removal of water from a synthesis gas conversion reactor such as an FT reactor. It would be further desirable to simultaneously provide for the addition of hydrogen at a controlled rate along the length of such a reactor to maintain sufficiently high hydrogen to carbon monoxide ratio to overcome the aforementioned current design constraints, thereby increasing the productivity of the reactor.